I found this Q&A which may help? The answer comes from Average Joe Restoration:
Car was restored 3 years ago and driven lightly since then. I then purchased it and immediately found out the Power steering was messed up although everything under the front end was all new, nos, oem and or rebuilt to oem.
But the power steering is very quiet and does a great job as long as I am turning to the right. But I have to use both hands as if it were manual steering when turning it to the left. Parking or on the road. I have had it in 6 different shops and spent a ton of money on it and still like it was. It never makes any noises idling and or on road. What can be wrong with it and what do I get done to it to fix it. Last shop said to replace it all, Pump, Ram, drag-link and complete control-valve assembly.
??
Answer:
The control Valve: the control valve determines which side of the ram cylinder gets pressurized to retract the rod or extend the rod from the cylinder.
So there are two possible problems: the cylinder may have a bad seal that is allowing the pressurized side fluid to pass to the return side or the control valve is failing and a seal in there is letting pressurized fluid pass or is restricting pressurized fluid from getting to the ram cylinder.
Here is the link:
http://averagejoerestoration.com/how-mustang-power-steering-works/Jeff